Modern armour-piercing projectiles are based on the principle of penetrating the armour under attack with high kinetic energy (KE) concentrated at a small area of the armour. The projectiles are subcalibre and designed as arrows with guiding fins. They have a length/calibre ratio which is 10:1 or higher. They are fired from guns with a calibre of at least 40 mm with muzzle velocities of 1500 m/s or more.
To achieve high KE the material in the projectile must be of high density. Normally, use is made of a heavy metal, e.g. a tungsten alloy containing a few percent of nickel and iron. Typically, the alloy consists of 92% tungsten, 5% nickel and 3% iron and has a density of 17.5 Mg/m.sup.3. The projectile material is produced from powder which is formed into rods and smelt-phase sintered at approx. 1470.degree. C. The production process is normally terminated by cold working and heat treating. Other projectile materials are impoverished uranium alloyed with titanium, but steel is also employed.
It is previously known in this art that armour-piercing projectiles are designed with cores of other material. For example, according to U.S. Pat. No. 4,616,569 of Oct. 14, 1986, an armour-piercing projectile is reinforced with a body extending throughout the entire projectile center and being of extreme strength and rigidity. The inner body, which at least in part consists of wires, is secured to the projectile by shrinking and serves to hold together the projectile on impact against the armour. According to U.S. Pat. No. 4,256,039 of Mar. 17, 1981, an axially extending core is provided with a wrapped foil of metallic glass (amorphous metal) of high hardness. By such means, there will be obtained a projectile with an outer portion of high strength. According to the present patent, the projectile is designed with a core of a different type, whose function is to reduce the resistance against penetration into the armour material.
On penetration of the projectile into steel armour of normal type, the tip of the projectile is gradually deformed at the same time as the material in the armour is displaced and a hole is formed, see FIG. 1. The penetration velocity into the armour will depend upon the KE of the projectile which is counterbalanced by the energy which is required to displace the armour material. If the point of contact between projectile and armour is regarded as stationary, the penetration may be described such that projectile and armour flow in towards the point of contact. From this, a pressure balance according to Bernoulli will be obtained: EQU 1/2p .sub.Pa U.sup.2 +R.sigma..sub.pa =1/2P.sub.p2 (V-U)+.sigma..sub.pr
wherein U is the velocity of the point of contact, V is the projectile velocity, p .sub.Pr is the density of the projectile, Pr, and p.sub.Pa is the density of the armour, Pa and .sigma. is the yield stress of each respective material. R is a geometric form factor which may be set at approximately=3.5.
The higher the velocity of the projectile, the higher the pressure at the contact surface between projectile and armour will be, and the higher the velocity will be at which the projectile and armour material are displaced out laterally. The radial material flow results in a penetration channel being formed in the armour. The higher the velocity of the radial material flow, the greater the diameter of the thus formed channel will be. At moderate projectile velocity (1500 m/s) the diameter of the thus formed hole will itself be itself moderate or about twice the diameter of the projectile. As the velocity increases, the channel becomes progressively wider. At velocities in excess of 2000 m/s, the KE which is consumed for the radial mass transport will be wholly predominant over the energy required to overcome the mechanical strength of the steel armour plating.
An increase in the mechanical strength of a projectile has only a limited effect on penetration. Moreover, the severe deformation of the projectile nose during penetration leads to such immense heat generation that the material locally melts and loses all mechanical strength. For an armour piercing projectile, substantial toughness is also required in order to be capable of penetrating several layers of modern armour plating. Normally, an increase in mechanical strength leads to a reduction in toughness.
At projectile velocities of less than 1000 m/s, hard projectiles (cemented carbides) are utilized, which retain their shape on penetration. For such projectiles, the material flow ahead of the penetrating projectile is influenced by the nose shape. A more acute--or spiculated--shape gives within certain limits lower resistance against penetration and thus deeper penetration. This is because the radial armour material displacement ahead of the penetrating projectile takes place at lower acceleration and lower velocity, whereby the resistance against penetration on account of the mass forces is reduced. In other words, it is possible to influence the penetration depth by the shape of the projectile nose. The original shape of the nose is obviously of no significance to armour-piercing projectiles which, at high velocity, are gradually deformed during armour penetration.
The possibilities of increasing penetration for armour-piercing projectiles are limited to increasing projectile velocity and the length/diameter ratio. However, such measures impose higher demands on the mechanical strength and toughness of the material in the projectile, something that is problematical to achieve.
A projectile shape which leads to lowered resistance to penetration by reduced mass forces is of importance, in particular since the trend in military technology is to raise projectile velocities to about 2000 m/s. At a higher velocity, the relative influence of the mass forces increases.